Probing Transient Nanoscale Electric and Magnetic Resonances of Dielectric Silicon Metasurfaces*

Controlling optical electromagnetic fields at nanoscale has gained interest over last decade as they improve device efficiency in energy, telecommunication and medical sector. Recently, it was demonstrated that dielectric nanostructures can also create strong electric and magnetic near fields similar to plasmonic nanostructures due to their Mie resonances. Unlike their metallic counterparts that can sustain mostly electric responses while having high ohmic losses, dielectric metasurfaces display rich optical features with low parasitic loss. Here we study the steady-state and femtosecond optical properties of dielectric silicon (Si) metasurfaces to understand how structural changes of the unit cell structure affect its overall optical performance. Si metasurfaces are fabricated using electron beam lithography and their scattering and absorption (extinction) properties are studied using white-light broadband spectroscopy. Furthermore, we perform ultrafast broadband transient absorption spectroscopy on these large arrays by exciting near (900 nm) and well above (350 nm) the Si bandgap to demonstrate how modulating the electron density dynamically modifies these electric and magnetic resonances.